Laser Beamed Interstellar Mission: A New Take

byPaul GilsteronOctober 2, 2008

For all their attraction as a way to leave weighty propellant behind, solar sails have a fundamental limitation. Their power source is the Sun. As you move away from the Sun, the amount of available light drops according to the inverse square law — a spacecraft that doubles its distance from the Sun encounters only a fourth of the sunlight previously available. Quadruple the distance and the sunlight drops to a sixteenth of what it was, making sail operations problematic in the outer Solar System.

And what of the stars? Solar sail specialist Greg Matloff has been juggling the numbers on interstellar travel via solar sail for decades now, and even with the best case scenario involving an extremely close solar pass, a thousand years to Centauri is about as good as it gets. And that’s quite a stretch in itself. Epsilon Eridani would actually make an easier mission as it’s much closer to the ecliptic, so you get 30 kilometers per second (Earth’s orbital velocity around the Sun) from scratch as you begin your solar approach. On the other hand, the resultant velocity (200 AU per year in one mission concept) takes 3500 years to reach Epsilon Eridani.

Laser propulsion is one way around the solar sail limitation, and as Matloff, along with co-authors Les Johnson and Giovanni Vulpetti, discuss in their new solar sail book, the method must be finely tuned for success. Earth-based lasers won’t do because of attenuation from Earth’s atmosphere, diminishing the beam’s intensity, and also causing it to diverge much more quickly. The result: Deeply compromised thrust on the sail. Earth’s rotation is also a major problem, making it impossible to keep the beam on the receding sail for extended periods.

Robert Forward pondered power stations in the inner Solar System to solve this problem, with the laser beam focused by a huge lens in the outer system for maximum effect. It’s interesting to see how that idea — created for its interstellar possibilities — has developed over the years. What Matloff, Johnson and Vulpetti talk about is a space-based laser in orbit around Jupiter. The orbital rotation problem is greatly eased because Jupiter orbits only once in twelve years, allowing ample time for beam adjustment and calibration. Not only that, but use a polar orbit and you can keep the sail under beam for a decade at a time.

And here’s where things get truly ingenious. Powering up that big laser could be handled by a tether, an idea dear to Robert Forward’s heart (he built an entire company, called Tethers Unlimited, around the concept). A long conducting wire deployed deep into Jupiter’s magnetosphere would generate a huge electrical flow. As the authors note, this is the same principle that is at work when an electrical generator produces electricity in a power plant. Wires moving through intense magnetic fields produce electricity, and Jupiter’s magnetic field is the second most powerful in the Solar System, second only to that of the Sun.

Forward envisioned the use of tethers in a much different way. Properly positioned, they could adjust spacecraft orbits and fling payloads around the Solar System without the need for rockets. But he would have loved the idea of using tethers for power generation around Jupiter, meeting the laser’s formidable needs. That would enable a beamed propulsion scenario capable of getting us into nearby interstellar space and shortening those lengthy travel times to Centauri and elsewhere.

From the book:

This is by no means the only scenario in which lasers might be used to push our sails. But it is certainly a likely one. A mission might proceed something like this: A sailcraft departs from Earth on a sunward bound trajectory. The craft falls toward the Sun and orients its sail to maximize solar thrust at perihelion, giving it an incredible boost toward the outer solar system. Sunlight continues to push on the sail until it reaches the orbit of Jupiter, at which point our tether-driven laser sends a beam of light to reflect from the sail, picking up from where the now-feeble sunlight leaves off. The laser maintains its aim point on the sail, providing continuous additional thrust, until the diffraction limit of the laser results in no net thrust being applied to the sail — somewhere in deep space…

There are, of course, alternatives to lasers when it comes to beamed propulsion. We’ll be talking soon with James Benford, the leading specialist on microwave beaming (I hope to have that interview up within the next couple of weeks), and particle beaming options using futuristic versions of a nuclear accelerator are also well worth considering. The point here is the flexibility of the sail itself. It’s a spacecraft concept that offers abundant applications right here in the Solar System, while holding out the promise of future adaptations that may well propel our first targeted star mission to its destination.

Comments on this entry are closed.

Paul F. DietzOctober 2, 2008, 16:14

Laser beaming offers interesting options not available to sails that use broadband light (like sunlight).

Laser light can be scattered resonantly off ions. Close to resonance, ions can have enormous scattering cross sections, and the ions can be contained in a magnetic field at temperatures far above those at which a solid sail would vaporize.

Another idea is to exploit laser cooling, in which scattering of the laser photons off an object can actually cool the object (by careful tuning of the laser to just under a resonance, so absorption occurs when some sail thermal energy pushes the absorber over the hump, so to speak). This does not violate any laws of thermodynamics, since the reemitted light has higher entropy than the incoming laser beam. It could potentially enable power densities at the sail far above those at which an unrefrigerated sail would be destroyed.

A few years ago there was a paper in the JBIS which used electron resonance to “reflect” microwaves and push a large interplanetary vehicle. I wonder if that might not work better than ions and lasers? Range is a bit limited by diffraction tho.

“…the Field Circus: a light sail craft that is speeding out of Sol system on a laser beam generated by Amber’s installations in low Jupiter orbit. (Superconducting tethers anchored to Amalthea drag through Jupiter’s magnetosphere, providing gigawatts of electricity for the hungry lasers: energy that comes in turn from the small moon’s orbital momentum.)”

The relevant chapter “Router” first appeared in the September 2002 issue of Asimov’s Science Fiction.

Just a thought… what happens to the beam that reflects off the back of the sail? Does it reflect back to Earth? Are provisions made to scatter the beam so that it doesn’t cause any damage on its return trip?

Instead of building one huge laser which would be hard to scale up in size, and prone to being a single point of failure for the mission, what if many smaller lasers were strapped to the heads of sharks genetically re-engineered to ‘swim’ in the atmosphere of Jupiter? You could tie a long copper wire to their tails to help generate the energy needed to power the laser. The sharks could feed on whatever indigenous life forms may inhabit the atmosphere of Jupiter, or the clouds of Jupiter could be seeded with lightweight shark bait as part of a recurring supply mission. Yes, yes, I know what you’re thinking, “how do we get the sharks to keep their lasers pointed toward the target?” That’s probably the easiest part of the whole plan: Pavlovian Conditioning.

Just a thought… what happens to the beam that reflects off the back of the sail? Does it reflect back to Earth?Unless the sail is a precisely shaped mirror (to within a fraction of a wavelength of the light), and unless it is aimed precisely at the Earth, that can not be a problem. In actuality, unless one were using Forward’s two stage deceleration scheme (in which a detached portion of the sail reflects the beam back to slow the ship), there is no reason to have a high optical quality reflector.

“…the Field Circus: a light sail craft that is speeding out of Sol system on a laser beam generated by Amber’s installations in low Jupiter orbit. (Superconducting tethers anchored to Amalthea drag through Jupiter’s magnetosphere, providing gigawatts of electricity for the hungry lasers: energy that comes in turn from the small moon’s orbital momentum.)”

Another reason for me to get farther into my stack of must-read books! Thanks for the pointer, and also the Analog citation.

With regard to these tethers generating energy. It’s enough of a problem as it is working out what kind of unobtainium could be used for a plain old space elevator tether capable of supporting any weight, even its own; how do we expect to be able to make superconducting tethers, when known superconductors have low critical temperatures and are made of metal alloys which don’t have the tensile strength to support their own weight on such a massive scale?

Just wanted to point out that the idea predates Accelerando by several years-to be exact, I described tether-powered laser/maser propulsion in detail in my 2002 novel Permanence, which Charlie has certainly read.

Karl, I’m remiss in not having mentioned Permanence, which is a great favorite of mine and the first of your books that I read. I’m going to go back into my copy to read through your description again.

Quote: “Wires moving through intense magnetic fields produce electricity”. Yes, and also a drag on the wires, which would pull down the satellite. This explanation of how to extract energy from Jupiter’s magnetic field is not convincing. If there is indeed a way of doing it, it deserves a better explanation.

Luigi: the tether could be attached to one of Jupiter’s inner moons. The energy would be from the gravitational potential energy being released as the moon spirals in closer to Jupiter. One could draw terawatts of power for many years before the orbits decay too much. Altermately, tethers could be attached to moons at supersynchronous altitude, in which case the energy comes from Jupiter’s rotation (and the moon is pushed outwards). The inner moons are moving faster and are immersed in a stronger magnetic field, however.

It seems everything is easy in principle, but daunting when ‘engineering’ has to go to work! Out of all the proposed technologies to do truly ‘fantastic’ feats of space travel… laser-driven light sails ‘seem’ the most feasible? (? for caution.)
Could you imagine millenia or a fraction of an eon from now… the entire Milky Way has the ‘equivalent’ of ‘a fiber optic’ transport network linking the star systems together. Vehicles that carry ‘passengers’ from one side of the galaxy to the other; going on journey’s at relativistic speed where ‘Centuries’ pass like days? And the strangest part of the ‘trip’ will be the return to your ‘origin’. Stop for a few ‘weeks’ at a ‘world here’ & ‘a system there’ & ‘a celestial hostile there’ …. meet aliens, robots, the broadband of the outer rim…. and come back 5 years later (275,000 +/- years on the Galactic Central timekeeper?) back to the Sol System.
Realistically… this scenario is as ‘probable’ as the late 19th Century fantasist ‘romanticize’ about ‘super coal fire dirigibles’ flying above the Martian continents.

These are the places (this forum) where you can wonder, What is it like ‘out there’? And how will we do all this? What an incredible life if humans will do such things.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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